Defining the calendar
If your fiscal year differs from the calendar year, the option enables you to select the start month of the fiscal year. The behavior of the fact data imports to Budgeting & Planning and Financial Consolidation depend on the int_RegularCalendarImport custom setting in EPM Administration. The option tells the import process what the starting month of the calendar is to enable the process to import the data correctly.
If the value of int_RegularCalendarImport is TRUE, then the periods always refer to the numbers of the calendar months. That is, Period 01 always refers to January, Period 02 refers to February, and so on. Thus, if May is the first period of your fiscal year, then May must be period 5 in the integration table from which you import May's data.